One of the problems with conventional sump pumps is that the pump motor is subject to damage if it continues to operate after it is no longer submerged in the cooling water of the sump. Such damage may be caused by sticky floats or defective or corroded switches which fail to provide a signal to the motor indicating that the sump level has dropped to a predetermined point at which the motor and pump can and should be turned off.
If, instead of the float type of level-indicating mechanism, a pressure or optical sensor is provided, prior designs of which applicant is aware have required the use of two such sensors, one to indicate that the water level has risen to a predetermined level at which pump operation is to begin and a second sensor to indicate that the level has subsequently dropped to a level at which pump operation can and should be terminated. It will be appreciated by those skilled in the art that it is desirable that there be two such vertically spaced control levels to reduce the frequency of on-off cycling of the pump in response to the fluctuating water level. However, the use of two sensors necessarily increases the cost of the control portion of the sump pump and complicates maintainability.
Accordingly, it is the principal object of the present invention to provide an improved sump pump design which both (1) provides an assured signal that the water in the sump has risen to a predetermined pump-activating level for turning on the pump motor, and (2) an assured signal that the water level has dropped to a point at which the motor can and should be turned off, and which provides a simple, reliable and economical control system for determining both the upper and lower control levels.